Incubation of proteins or lipids with aldose sugars results in nonenzymatic glycation and oxidation of amino groups on proteins to form Amadori adducts. Over time, the adducts undergo additional rearrangements, dehydrations, and cross-linking with other proteins to form complexes known as Advanced Glycosylation End Products (AGEs). Factors which promote formation of AGEs included delayed protein turnover (e.g. as in amyloidoses), accumulation of macromolecules having high lysine content, and high blood glucose levels (e.g. as in diabetes) (Hori et al., J. Biol. Chem. 270:25752-761, (1995)). AGEs have been implicated in a variety of disorders including complications associated with diabetes and normal aging.
AGEs display specific and saturable binding to cell surface receptors on monocytes, macrophages, endothelial cells of the microvasculature, smooth muscle cells, mesengial cells, and neuroris. The Receptor for Advanced Glycated Endproducts (RAGE) is a member of the immunoglobulin super family of cell surface molecules. The extracellular (N-terminal) domain of RAGE includes three immunoglobulin-type regions: one V (variable) type domain followed by two C-type (constant) domains (Neeper et al., J. Biol. Chem., 267:14998-15004 (1992); Schmidt et al., Circ. (Suppl.) 96#194 (1997)). A single transmembrane spanning domain and a short, highly charged cytosolic tail follow the extracellular domain. The N-terminal, extracellular domain can be isolated by proteolysis of RAGE to generate soluble RAGE (sRAGE) comprised of the V and C domains.
RAGE is expressed in most tissues, and in particular, is found in cortical neurons during embryogenesis (Hori et al., J. Biol. Chem. 270:25752-761 (1995)). Increased levels of RAGE are also found in aging tissues (Schleicher el at., J. Clin. Invest., 99 (3): 457-468 (1997)), and the diabetic retina, vasculature and kidney (Schmidt et el., Nature Med., 1:1002-1004 (1995)). Activation of RAGE in different tissues and organs leads to a number of pathophysiological consequences. RAGE has been implicated in a variety of conditions including: acute and chronic inflammation (Hoffman et al. Cell 97:889-901 (1999)) the development of diabetic late complications such as increased vascular permeability (Wautier et al., J. Clin. Invest., 97:238-243 1996, nephropathy (Teillet et. al., J. Am. Soc. Nephrol., 11:1488-1497 (2000), atherosclerosis (Vlassara et. al., The Finnish Medical Society DUODECIM. Ann. Med., 28:419-426 (1996), and retinopathy (Hammes et al. Diabetologia, 42:603-607 (1999)). RAGE has also been implicated in Alzheimer's disease (Yan et al., Nature, 382:685-691 (1996)); erectile dysfunction; and in tumor invasion and metastasis (Taguchi et al., Nature, 405:354-357 (2000)).
In addition to AGEs, other compounds can bind to, and modulate RAGE. In normal development, RAGE interacts with amphoterin, a polypeptide which mediates neurite outgrowth in cultural embryonic neurons (Hori et. al., 1995). RAGE has also been shown to interact with calgranulin-like ligands and β-amyloid (Yan et. al., Nature, 389:689-695, (1997); Yan et al., Nature, 382:685-691 (1996); Yan et al., Proc. Natl. Acad. Sci. 94:5296-5301 (1997)).
Binding of ligands such as AGEs, S100/calgranulin, β-amyloid, CML (Nε-Carboxymethyl lysine), and amphoterin to RAGE has been shown to modify expression of a variety of genes. For example, in many cell types interaction between RAGE and its ligands generates oxidative stress, which thereby results in activation of the free radical sensitive transcription factor NF-κB, and the activation of NF-κB regulated genes, such as the cytokines IL-1β, TNF-α, and the like. In addition, several other regulatory pathways, such as those involving p21ras, MAP kinases, ERK1 and ERK2, have been shown to be activated by binding of AGEs and other ligands to RAGE. In fact, transcription of RAGE itself is regulated at least in part by NF-κB. Thus, an ascending, and often detrimental, spiral is fueled by a positive feedback loop initiated by ligand binding. Antagonizing binding of physiological ligands to RAGE, therefore, is a logical target for down-regulation of the pathophysiological changes brought about by excessive concentrations of AGEs and other ligands for RAGE.
Thus, there is a need for the methods to discover compounds which modulate binding of physiological ligands to the RAGE receptor. The method should comprise an assay which is sensitive and yet highly specific. Preferably, the method should utilize RAGE, or the binding site of RAGE itself, thereby allowing the development of modulators which vary in structure and physiological effect. Ideally, the method should lend itself to high throughput, automative techniques suitable for large scale screening of multiple compounds.